A space vehicle such as a satellite carries numerous pieces of electronic equipment such as amplifiers, receivers, computers, . . . . These pieces of equipment are often provided redundantly for reasons of reliability, and they can belong for example to a satellite relay system for television or telephone signals, or to systems for performing service functions on board the satellite (on-board management, attitude control, power supply, etc. . . . ).
When a satellite is launched, a nominal operating configuration is selected, i.e. certain pieces of equipment are selected to perform service functions, and a certain number of channels for transmitting telephone signals, and a certain number of channels for transmitting TV signals. During the lifetime of the satellite, and in particular for commercial reasons, it can happen that the configuration needs to be modified. In order to reconfigure the equipment to satisfy demand, ground control and monitoring stations send transmission control (TC) signals by radio which are received by a receiver on board the satellite, which then applies these signals both to a digital control unit (CU) constituted by a control terminal unit (CTU) which processes the signals, and to a plurality of remote terminal units (RTUs) for forwarding the signals to the various pieces of equipment. These TC signals can also be used to select among redundant pieces of equipment those pieces which are to be active, in particular in systems for performing service functions on board the satellite. The TC signals thus serve to control and manage the overall operation of the electronic equipment on board the satellite.
It is also always desirable to perform monitoring tests, either to verify that the TC signals have been properly executed, or to detect on the ground any breakdowns that might impede proper operation of the satellite or disturb the signals it relays. The various pieces of equipment on board the satellite therefore send telemetry (TM) signals to the central unit, which signals are then forwarded to the ground by appropriate transmitters located on board the satellite and in communication with the central unit. The TM signals can be transmitted either in response to TC signals, or else in systematic manner in order to monitor the state of the on-board equipment on a continuous basis.
The transmission of TM and TC signals between the central unit and the various pieces of on-board equipment is conventionally performed over wire links using cables; the set of wire links for carrying remote control and telemetry signals being referred to as the “TM-TC harness”.
It is always of great importance when designing a satellite to reduce on-board mass to as small a value as possible in order to reduce the cost of launching the satellite and also the cost of fuel needed to maintain the satellite on station in orbit, or indeed to increase the length of time a satellite can be maintained on station in orbit for a given quantity of on-board fuel.
Unfortunately, the TM-TC harness represents considerable mass (several tens of kilograms). The wire links it provides are numerous since each one of them must be a both-way link between the piece of equipment in question and the central unit, in order to keep TM and TC signals separate from the signals relating to each of the pieces of equipment. Furthermore, the cost of such a harness is high because of the time taken to design the harness and the time required to test and verify each wire and to connect it. In addition, the cables used for making such links are usually shielded cables in order to avoid disturbances due to interfering electromagnetic radiation.
Conventional solutions for reducing harness mass and the number of links exist in two forms: there are wire buses for reducing the number of wires, and there are wireless links for omitting wires.
The principle of the bus is to combine the previously digitized TM/TC signals in a time-division multiplex and to carry them in series at a high rate (approximately 1 million bits per second (Mb/s)) over a wire bus. The few wires of the bus are connected to all of the various pieces of equipment in a “daisy chain”. That solution does not eliminate the harness completely and it requires the interconnected pieces of equipment to operate at high bit rates. Problems of electromagnetic compatibility increase the amount of shielding needed and therefore increase the mass of the harness. Problems associated with breakdown propagation make redundancy schemes more complicated. Finally, the terminal modules at the end of the bus link in each piece of equipment are complex and greatly overdimensioned for the volume of TM/TC signals that normally required for ordinary pieces of microwave equipment (e.g. for a channel amplifier or a frequency converter).
The principle of wireless connections lies in using either radiowaves or infrared light (IR) for carrying signals by radiating them through free space.
Radiofrequency links make problems of electromagnetic compatibility considerably more complex (whether with the equipment itself, or with test benches, or with the satellite, or with the launcher, or with the launching pad). Characterizing a transmission channel is very difficult (it must be done inside a Faraday cage cluttered with masses of metal) and varies with the degree of integration of the satellite (flat panels, integrated communications module, satellite in its flight configuration).
When transmitting TM/TC signals using infrared light, one system using such a principle is described in French patent No. 2 696 890 filed by Alcatel Espace. In spite of their multiple advantages, infrared links present problems with shadow zones and they require transmitters and receivers to be optically visible to one another, which puts a constraint on how the satellite is arranged. In addition, the bit rates that can be used depend to a great extent on the geometry of the system, it being understood that this geometry varies depending on the degree of integration of the satellite.